The purpose of this study was to correlate major chronologic events known to have a bearing on the risk for developing breast carcinoma with histologic tumor type. Data were obtained for 1024 consecutively treated patients with the seven most common forms of mammary carcinoma. The age of occurrence of the following events was determined: menarche; first marriage; first pregnancy and first childbirth; last pregnancy and last childhood; diagnosis. The interval between each of these events and diagnosis of breast cancer was also calculated. Patients with medullary carcinoma had a significantly lower mean age at diagnosis than those with any of the other tumor types. Because of the lower mean age at diagnosis, medullary carcinoma patients were found to have significantly shorter intervals between age at diagnosis and the following: menarche, first pregnancy or first childbirth, and last pregnancy or last childbirth. Intervals between age at diagnosis and these other events were not significantly different among patients with other types of carcinoma. The ages of menarche, marriage, or pregnancy were not related independently to any tumor type. These data indicate that except for current age, the age of occurrence of major risk factors for developing breast cancer is unrelated to the histological type of tumor a patient develops.